An uplink Sounding Reference Signal (SRS) is an uplink signal transmitted from a mobile terminal, and a base station performs operations of uplink timing detection, power control, uplink scheduling in the frequency domain, link adaptation, etc., according to uplink channel status information provided in the SRS upon reception of the SRS. For a Time Division Duplex (TDD) system, when Multiple Input Multiple Output (MIMO) transmission based on preprocessing is used for downlink transmission, the base station may also perform channel estimation from the SRS and further derive downlink Channel State Information (CSI) due to the symmetry of the uplink and downlink channels.
Before the mobile terminal transmits an SRS to the base station, the base station needs to configure a radio frequency transmission module of the mobile terminal with some parameters and send these parameters to the mobile terminal via signaling, and the mobile terminal configures the radio frequency transmission module with these parameters upon reception of the parameters, so that the radio frequency transmission module may transmit the SRS over a transmission antenna according to the parameters configured by the base station. Upon reception of the SRS transmitted from the mobile terminal, the base station can parse the SRS to derive the parameters thereof and determine from the parameters derived from the SRS the mobile terminal transmitting the SRS.
Typically, the base station configures the radio frequency transmission module of the mobile terminal with the parameters including a duration, a cycle and a specific symbol location for transmitting an SRS, a frequency domain location and a frequency hop pattern for transmitting the SRS, and a Transmission Comb (TC) value and a Cyclic Shift (CS) value.
The inventors of the invention have identified that existing mobile terminals provided with two or more transmission antennas cannot support concurrent transmission of SRSs over the transmission antennas and have to transmit the SRSs alternatively. FIG. 1 illustrates a schematic diagram of transmitting SRSs from a mobile terminal with two transmission antennas to a base station in the prior art in a specific transmission flow as illustrated in FIG. 2 including the following steps.
S200: the base station derives the number of transmission antennas of the mobile terminal.
The base station can derive the number of transmission antennas of the mobile terminal when the mobile terminal gains an access thereto.
S201: the base station assigns the mobile terminal with various parameters for transmitting SRSs.
The base station assigns the mobile terminal with various parameters for transmitting SRSs including a duration, a transmission cycle and a specific symbol location for transmitting an SRS, a frequency domain location and a frequency hop pattern for transmitting the SRS, and a TC value and a CS value.
S202: the base station transmits a result of assignment to the mobile terminal via dedicated signaling.
S203: upon reception of the result of assignment transmitted from the base station, the mobile terminal transmits an SRS to the base station according to the result of assignment over a transmission antenna 1 upon detection of arrival of an SRS transmission cycle (called a transmission cycle for short hereinafter).
The mobile terminal transmits an SRS to the base station in a transmission cycle which typically may be 2 ms, 5 ms, 10 ms, 20 ms, 40 ms, 80 ms, 160 ms, 320 ms, etc. In other words, the mobile terminal which has transmitted an SRS transmits another SRS at an interval of one transmission cycle. The mobile terminal transmits the SRS to the base station over the transmission antenna 1 according to the result of assignment, i.e., the assigned parameters including the TC value and the CS value, sent from the base station.
S204: the mobile terminal transmits an SRS to the base station over a transmission antenna 2 according to the result of assignment upon detection of arrival of another transmission cycle.
The transmission antenna 2 is configured with the same parameters as those configured in the previous step S203 for the transmission antenna 1, and both of the transmission antennas 1 and 2 transmit the SRSs to the base station according to the assigned parameters (including the TC value and the CS value) sent from the base station.
The foregoing steps S203 and S204 are repeated, so that the SRSs are transmitted over the two transmission antennas of the mobile terminal in different transmission cycles alternatively at the same time and frequency domain location and with the same TC value and CS value, and the base station determines, from the number of transmission antennas of the mobile terminal, over which corresponding one of the transmission antennas the SRSs received in any of the different transmission cycles is transmitted, for example, the SRSs are transmitted over the transmission antenna 1 upon arrival of transmission cycles 1, 3, 5, 7 . . . and over the transmission antenna 2 upon arrival of transmission cycles 2, 4, 6, 8 . . . , and then the base station determines from the number (i.e., 2) of transmission antennas that the SRSs received in the transmission cycles 1, 3, 5, 7 . . . are transmitted over the transmission antenna 1 and the SRSs received in the transmission cycles 2, 4, 6, 8 . . . are transmitted over the transmission antenna 2. Such transmission mode of SRSs over multiple transmission antennas of a mobile terminal in the prior art is also referred to as Time Division Multiplexing (TDM) mode.
The inventors of the invention have identified that SRSs can not be transmitted concurrently (i.e., in the same transmission cycle) over multiple transmission antennas of a mobile terminal in the prior art, and a cycle in which an SRS is transmitted over one of the transmission antennas is essentially equivalent to N transmission cycles, where N is equal to the number of the transmission antennas of the mobile terminal. In other words, an SRS may be transmitted again to the base station over a transmission antenna of the mobile terminal only after two or more transmission cycles. Since an SRS is transmitted over a transmission antenna at a too long interval, the base station typically can not know accurately uplink channel status information of respective transmission antennas from the transmitted SRSs and consequently can not perform accurate channel estimation on the respective transmission antennas.